1. Field of the Invention
The present invention relates to a union joint and, more particularly, to a union joint for use in semiconductor industries requiring perfectly sealed connection of pipes.
2. Prior Art
Semiconductor device manufacturing processes for manufacturing semiconductor devices, such as ICs and LSIs, are carried out in clean rooms in which elaborate precautions are employed to reduce dust particles and other contaminants in the air. A special gas for use in the clean room is supplied through piping from an external gas source. The special gas must be extremely pure and the existence of foreign gasses, such as air, in the special gas must be perfectly obviated. Therefore, the piping for supplying the special gas must be formed of precision pipes and precision union joints for connecting the pipes. Union joints to be used in combination with high-pressure vessels also must be capable of perfect sealing in view of safety.
The perfectly sealed connection of pipes by a conventional union joint may be attained by increasing pressure acting on a metallic gasket provided between the pipes. However, if an excessive torque is applied to the coupling ring to join pipes firmly, the pipes and a gasket interposed between the pipes are distorted torsionally, and hence the torque cannot be increased beyond a fixed limit.
The applicant of the present patent application proposed previously a union joint as shown in FIG. 4 incorporating a thrust bearing to transmit only an axial force from the inner surface of a coupling nut to one of two pipes and not to transmit torque from the inner surface of the coupling nut to the pipe in Japanese Patent Laid-Open Publication No. 62-75188. As shown in FIG. 4, the union joint consists of a first sleeve 10 attached in a sealed joint to a first pipe 1, a second sleeve 20 attached in a sealed joint to a second pipe 2, a metallic gasket 30 interposed between the pressing surface 15 of the first sleeve 10 and the pressing surface 25 of the second sleeve 20, a coupling nut 5 for axially drawing together the first sleeve 10 and the second sleeve 20 to join the first pipe 1 and the second pipe 2 hermetically, and a thrust bearing 8 placed between the inner bottom surface 6 of the coupling nut 5 and the shoulder of the first sleeve 10. When the coupling nut 5 is turned to draw together the first sleeve 10 and the second sleeve 20, hence the first pipe 1 and the second pipe 2, only an axial force acts on the first sleeve 10, so that the pipes 1 and 2 can be firmly joined together by applying a large torque to the coupling nut 5 without torsionally distoring the pipes 1 and 2 and the metallic gasket 30. In order to enhance the sealing effect of the union joint, the pressing surface 15 of the first sleeve 10 and the pressing surface 25 of the second sleeve 20 are mirror-finished and, to enhance the possibility of repetitive use, the pressing surfaces 15 and 25 are burnished. The metallic gasket 30 having excellent squeezing property, capable of securing residual elasticity, capable of functioning without contaminating the gas and having excellent durability can be used repeatedly to meet economical requirements
To reserve an appropriate residual elasticity of the metallic gasket 30, the compression of the metallic gasket 30 between the sleeves 10 and 20 must be within a limited range. To limit the compression of the metallic gasket 30, the sleeves 10 and 20 are provided respectively with limiting surfaces 16 and 26. To locate the metallic gasket 30 correctly between the sleeves 10 and 20, a gasket chamber S surrounded by a side wall 28 is formed in the second sleeve 20. In a sate shown in FIG. 4 before the coupling nut 5 is turned to fasten together the sleeves 10 and 20, the metallic gasket 30 is placed in the gasket chamber S and seated on the pressing surface 25 formed in the second sleeve 20, and the pressing surface 15 of the first piece is positioned opposite to the metallic gasket 30 in the gasket chamber S. When the coupling nut 5 is turned to draw the sleeves 10 and 20 axially toward each other, the metallic gasket 30 is compressed between the pressing surfaces 15 and 25 until the limiting surfaces 16 and 26 are brought into contact with each other.
This union joint, however, still needs further improvements. Since the pressing surface 15 of the first sleeve 10 is the end surface of a protruding portion of the first sleeve 10, the pressing surface 15 can be quickly and easily burnished. However, since the pressing surface 25 of the second sleeve 20 is the bottom surface of the recessed gasket chamber S, it is considerably difficult to burnish the pressing surface 25 and burnishing the pressing surface 25 increases the cost of the union joint. Since the metallic gasket 30 expands radially when compressed and the outer circumference of the metallic gasket 30 approaches the side surface 28 of the gasket chamber S, the entire area of the pressing surface 25 including the peripheral area contiguous with the side surface 28 must be burnished, which, however, is very difficult. Means for locating the metallic gasket 30 in a gasket holding space S' on the circumference 28' of the protruding end of the second sleeve 20 as shown in FIG. 7 also has the same problem in burnishing the pressing surface 25 on which the metallic gasket 30 is seated.
The union joint has problems in assembling and disassembling. In axially separating the sleeves 10 and 20 in a space where the pipes 1 and 2 cannot be axially moved after screwing the coupling nut 5 off the second sleeve 20, the pressing surfaces 15 and 25 need to be moved laterally with respect to an axis P, which, however, is impossible because part of the protruding end 18 of the first sleeve 10 is fitted in the gasket chamber S.
In some cases, minute radial flaws 50 as shown in FIGS. 5 and 6 are formed across a portion of the pressing surfaces 15 (25) to be in contact with the metallic gasket 30. Such a radial flaw 50 makes impossible to secure perfect sealing however great is the pressure applied to the metallic gasket 30, because the metallic gasket 30 does not cut into the radial flaw 50 and expands radially after the pressure applied to the metallic gasket 30 exceeds a certain level.